1. Field of the Invention
The present invention relates to switching power supply devices that include switching elements and switching control circuits. In particular, the present invention makes it possible to apply a general purpose current mode IC to a high performance power conversion circuit.
2. Description of the Related Art
FIG. 1 is a circuit diagram of a switching power supply device described in Japanese Unexamined Patent Application Publication No. 2001-37220. In FIG. 1, a switching power supply device 1 is based on a flyback converter circuit, and a primary switching element Q1 is turned on and off in an alternating manner. When the primary switching element Q1 is on, energy is accumulated in a transformer T, and when the primary switching element Q1 is off, power is supplied to a load. In addition, the switching power supply device 1 employs a so-called voltage clamp system, in which a surge voltage that is applied to the primary switching element Q1 is clamped, and this achieves a zero voltage switching operation of the primary switching element Q1 and a secondary switching element Q2.
Specifically, in the switching power supply device 1, an FET Q1, which serves as the primary switching element, a primary winding N1 of the transformer T, and a direct current power supply E are connected in series, and a series circuit formed by an FET Q2, which serves as the secondary switching element, and a capacitor C1 is connected between the two ends of the primary winding N1 of the transformer T.
Here, a gate of the FET Q1 is connected to one end of a first drive winding N3 via a switching control IC 2. In addition, a source of the FET Q2 is connected to a drain of the FET Q1, and a gate of the FET Q2 is connected to one end of a second drive winding N4 of the transformer T via a secondary switching element control circuit (secondary control circuit) 3.
Furthermore, the gate and the source of the FET Q2 are connected between the two ends of the second drive winding N4 via the secondary control circuit 3. The secondary control circuit 3 includes a transistor Q3, a capacitor C2, a resistor R1, a capacitor C3, a resistor R2, and an inductor 4. Of these components, the capacitor C2 and the resistor R1 form a time constant circuit.
In addition, the switching power supply device 1 includes a rectifying diode Do and a smoothing capacitor C4, which are provided at a secondary side of the transformer T.
If the switching control IC 2 as illustrated in FIG. 1 is prepared for each of the variety of applications in a power conversion circuit and a different IC is used for a different specification or a different purpose, a large variety of ICs become necessary with an increase in the number of applications. The development and the manufacture of an individual IC require considerable processes and costs. The increase in the variety also complicates the logistics and the inventory management of the ICs, which results in a problem that the unit cost of the ICs increases.
In particular, in a current resonance type power conversion circuit having a half bridge configuration that uses two switching elements or in an insulating type power conversion circuit having a power factor correction function (PFC converter), the waveform of a current that flows in the power conversion circuit does not necessarily take on such a waveform in which a current value increases monotonously in proportion to time during an on period in which the electricity runs in the switching element, which leads to an issue that a general purpose current mode IC cannot be used.